Sintered steel bonded titanium carbide tool steel characterized by an improved combination of transverse rupture strength and resistance to thermal shock

ABSTRACT

A SINTERED STEEL-BONDED TITANIUM CARBIDE COMPOSITION CHARACTERIZED BY AN IMPROVED COMBINATION OF TRANSVERSE RUPTURE STRENGTH, RESISTANCE TO IMPACT, AND RESISTANCE TO THERMAL SHOCK IS PROVIDED COMPRISING BY WEIGHT ABOUT 20% TO 30% OF PRIMARY GRAINS OF TITANIUM CARBIDE DISPERSED THROUGH A STEEL MATRIX MAKING UP THE BALANCE, THE COMPOSITION OF SAID MATRIX CONSISTING ESSENTIALLY BY WEIGHT OF ABOUT 3% TO 7% CHROMIUM, ABOUT 2% TO 6% MOLYBDENUM, ABOUT 0.1% NICKEL, ABOUT 0.3% TO 0.7% CARBON AND THE BALANCE ESSENTIALLY IRON. IN THE HEAT TREAYED HARDENED CONDITION, THE MATRIX IS CHARACTERIZED BY THE PRESENCE OF MARTENSITE.

United States Patent 6 SINTERED STEEL BONDED TITANIUM CARBIDE TOOL STEEL CHARACTERIZED BY AN IM- PROVED COMBINATION OF TRANSVERSE RUP- TURE STRENGTH AND RESISTANCE TO THERMAL SHOCK M. Kumar Mal, Spring Valley, and Stuart E. Tarkan, Monsey, N.Y., assignors to Chromalloy American Corporation, West Nyack, N.Y. No Drawing. Filed Dec. 29, 1972, Ser. No. 319,151

Int. Cl. C22c l/05, 29/00 US. Cl. 29182.8 4 Claims ABSTRACT OF THE DISCLOSURE A sintered steel-bonded titanium carbide composition characterized by an improved combination of transverse rupture strength, resistance to impact, and resistance to thermal shock is provided comprising by weight about 20% to 30% of primary grains of titanium carbide dispersed through a steel matrix making up the balance, the composition of said matrix consisting essentially by weight of about 3% to 7% chromium, about 2% to 6% molybdenum, about 0.1% to 1% nickel, about 0.3% to 0.7% carbon and the balance essentially iron. In the heat treated hardened condition, the matrix is characterized by the presence of martensite.

This invention relates to a sintered steel-bonded titanium carbide composition and to a hardened wear resistant element produced from said composition, said sintered steel-bonded composition being characterized by an improved combination of physical properties, including improved transverse rupture strength, improved resistance to thermal shock, and improved resistance to impact and the like properties.

STATE OF THE ART Titanium carbide tool steel compositions are disclosed in US. Pat. No. 2,828,202 (assigned to the same assignee) comprising broadly primary grains of essentially titanium carbide distributed through a heat treatable steel matrix. A typical composition is one containing by weight 33% TiC in the form of primary carbide grains dispersed through a steel matrix, the steel matrix containing by weight 3% Cr, 3% M0, 0.6% C and the balance essentially iron. The steel is preferably produced using powder metallurgy methods which comprise broadly mixing powdered titanium carbide (primary carbide grains) with powdered steel-forming ingredients of, for example, the aforementioned composition, forming a compact by pressing the mixture in a mold and then subjecting the compact to liquid phase sintering under non-oxidizing conditions, such as in a vacuum. The term primary carbide employed herein is meant to cover the titanium carbide grains per se added directly in making up the composition and which grains are substantially unaffected by heat treatment.

In producing a titanium carbide tool steel composition containing, for example, about 33% by weight of TiC (approximately 45 volume percent) and substantially the balance a steel matrix, about 500 grams of TiC (of about 5 to 7 microns in size) are mixed with 1000 grams of steel-forming ingredients in a mill half filled with stainless steel balls. To the powder ingredients is added one gram of paraffin wax for each 100 grams of mix. The milling is conducted for about 40 hours, using hexane as a vehicle.

After completion of the milling, the mix is removed and dried and compacts of a desired shape pressed at about 15 t.s.i. and the compacts then subjected to liquid phase sintering in vacuum at a temperature of about 2640 F.

(1450 C.) for about one-half hour at a vacuum corresponding to 20 microns of mercury or better. After completion of the sintering, the compacts are cooled and then annealed 'by heating to about 1650 F. (900 C.) for 2 hours followed by cooling at a rate of about 27 F. (15 C.) per hour to about 212 F. (100 C.) and thereafter furnace cooled to room temperature to produce an annealed microstructure containing spheroidite. The annealed hardness is in the neighborhood of about 45 R and the high carbon tool steel is capable of being machined and/or ground into any desired tool shape or machine part prior to hardening.

The hardening treatment comprises heating the machined piece to an austenitizing temperature of about 1750 F. for about one-quarter hour followed by quenching in oil or water to produce a hardness in the neighborhood of about 70 R While the foregoing typical composition has achieved some measure of commercial success, it has certain disadvantages. For example, when used as die material, under conditions in which heat is generated due to friction, or where the metal being worked upon has been preheated, over-tempering tended to occur, leading to softening of the die steel. In addition, unless care was taken to avoid rapid heating and cooling, a part made of the composition would be subject to thermal cracking. Moreover, the transverse rupture strength, while adequate for most uses, was not as high as desired, the transverse rupture strength usually ranging from about 250,000 p.s.i. to about 300,000 p.s.i.

Another type of steel-bonded carbide is that disclosed in US. Pat. -No. 3,653,982 (also assigned to the same assignee), a typical commercial composition being one containing by weight about 34.5% TiC as primary carbide grains dispersed through a steel matrix making up essentially the balance. The steel matrix contains by weight based on the matrix itself about 10% Cr, 3% Mo, 0.85% C and the balance essentially iron. This steelbonded carbide dilfers from the aforementioned lowerchromium variety in that it is capable of being tempered at about 1000 F. (538 C.) and thus is capable of retaining fairly high hardness at such temperatures, particularly when used as an apex wear resistant seal strip in rotary piston engines, such as the Wankel engine. However, this composition, like the previously discussed composition is subject to thermal shock and usually exhibits a transverse rupture strength ranging from about 250,000 p.s.i. to 300,000 p.s.i.

A steel-bonded carbide composition which exhibits resistance to softening at elevated temperatures is one covered by US. Pat. No. 3,053,706 (also assigned to the same assignee). A typical composition is one in which the refractory carbide is a solid solution carbide of the type WTiC containing about WC and 25% TiC. This carbide, preferably in an amount by weight of 45.6%, is dispersed through a steel matrix making up essentially the balance. The matrix which is capable of secondary hardening at 1000 'F. to 1200 F. (538 C. to 650 C.) typically may contain 12% W, 5% Cr, 2% V, 0.85% C and the balance essentially iron. The dissolved tungsten in the matrix is in equilibrium with the saturated solution of the primary carbide. While the foregoing composition is satisfactory in providing the necessary secondary hardening effect to resist tempering at warm die-working temperatures, these compositions tended to be porous. For example, as pointed out in column 4 of the patent, lines 4 to 9, the composition was satisfactory in producing a sintered slug one-half inch thick. However, it was subsequently found that in producing large sizes for use in dies, for example, sizes of about 1 /2 inches square and larger, the finally sintered product tended to be porous. In addition, the transverse 3 V rupture strength was not all that was desired, the transverse rupture ranging from about 220,000 p.s.i. to 250,000

Tooling and component part manufacturers have been constantly seeking newer and better materials capable of "withstanding stresses, thermal shock, impact, heat and wear encountered in certain hot work and impact-involving applications, such as warm heading dies, swedging dies, forging dies, die casting tools, and the like. This demand has created an urgent need for steel-bonded titanium carbide material having a unique combination of physical and mechanical properties at room and elevated temperatures, particularly improved resistance to impact and improved transverse rupture strength in combination with improved resistance to thermal shock.

We have found that the foregoing can be achieved by employing a titanium carbide tool steel composition in which the steel matrix is characterized by a judicious proportioning of the steel-forming ingredients chromium,

' molybdenum, nickel and carbon.

OBJECTS OF THE INVENTION STATEMENT OF THE INVENTION Stating it broadly, the invention resides in a sintered steel-bonded titanium carbide composition containing by weight about 20% to 30% of primary grains of titanium carbide dispersed through a steel-matrix making up essentially the balance of about 80% to 70%, said matrix consisting essentially by weight of about 3% to 7% chromium, about 2% to 6% molybdenum, about 0.1% to 1% nickel, about 0.3% to 0.7% carbon and the balance-essentially iron.

A preferred composition is one containing 24% to 30% titanium carbide and the balance essentially the steel matrix of about 76% to 70%, the steel matrix preferably consisting essentially by weight of about 4% to 6% chromium, about 3% to molybdenum, about 0.25% to 0.75% nickel, about 0.3% to 0.5% carbon and the balance essentially iron.

It has been found that by adding nickel over controlled ranges to the matrix, markedly improved resistance to thermal shock is obtained combined with markedly improved transverse rupture strength.

A preferred composition is one containing by weight approximately 25% TiC and approximately 75% the steel matrix, the matrix consisting essentially by weight of about 5% Cr, about 4% Mo, about 0.5% Ni, about 0.4% C and the balance essentially iron.

By employing nickel in the matrix over the range of about 0.1% to 1% and, preferably over the range of about 0.25% to 0.75%, transverse rupture strengths of over 325,000 p.s.i. and even over 350,000 p.s.i. are obtained, for example, transverse rupture strengths in the range of about 400,000 p.s.i. to as high as 550,000 p.s.i., agcolrglpanied by markedly improved resistance to thermal s cc As illustrative of the improved results obtained with the invention, the following example is given.

EXAMPLE roduced as follows.

About 1,000 grams of titanium carbide'povvder of about 5 to 7 microns average size are mixed with 3,000 grams of steel-forming ingredients of the foregoing composition of 20 microns average size in a steel ball mill (stainless steel balls). The carbon added to the mix takes into account any free carbon in the titanium carbide raw material. To the mix is added one gram of parafiin wax for each grams of mix. The milling is conducted for about 40 hours with the mill half full of steel'balls of about one-half inch in diameter using hexane as the vehicle. A

After completion of the milling, the mix is removed and vacuum dried. A predeterminedamount of the. mixed powder is compressed in a die at about ,15 tonsper square inch (t.s.i.) to the desired shape. The shape is liquid phase sintered, that is, sintered above the melting .point of the matrix composition, at a temperature of about 1435 C. for one hour in vacuum, e. g.,. a vacuum corresponding to 20 microns of mercury or better. After completion of sintering, the shape is cooled and then annealed by heating to 900 C. for 2 hours followed by cooling at a rate of about 15 C. per hour to about 100 C."to produce an annealed microstructure containing "spheroidite, the annealed hardness of the composition falling within the range of about 40 to 50 R A comparison was made using a difl'erent steel matrix made of a chromium-type hot work tool steel containing by weight 5% Cr, 1.5% M0, 1.5% W, 0.4% V, 0.35% C and the balance essentially iron.

Both carbide compositions contained 25% by weight of TiC and 75 by weight of the steel matrix.

Each of the compositions in the annealed state were oil quenched after 30 minutes at 1875 F. (1025", C.). The compositions were double tempered for one hour each at 975 F. (525 C.) and air cooled. i v

Tranverse rupture tests were conducted on etich and the results obtained are as follows:

1 Transverse Rupture Strength.

As will be noted from the foregoing data, the carbide steel alloy of the invention exhibits almost doublethe transverse rupture strength compared to the carbide .steel alloy outside the invention. v n n In the hardened state, the carbide alloys are. characterized by a microstructure of essentially marten site. Q

In addition to the aforementioned example, additional tests were conducted on commercial sintered cornpositions referred to hereinbefore which include the following by weight: i (A) 33% TiC-67% steel matrix; matrix-3% Cr, "3%

M0, 0.6% C and the balance essentially Fe. (B) 34.5% TiC-65.5% steel matrixf matrix-10%" c1,

3% Mo, 0.85% C and the balance essentially Fe. c 45.6% wTic,-s4.4% steel matrix writ: "solid solution containing 75 WC-25%TiC);"n1'at'rix -I1i2% W, 5% Cr, 2% V, 0.85 C and balance essentially Fe.

The foregoing compositions were produced by sintering similarly to the carbide steel alloy of the invention described herein. These compositions were compared to the preferred alloy of the invention comprising 25% TiC and 75% steel matrix (5% Cr, 4% M0, 0.5% Ni, 0.4% C and 'balance essentially Fe). All compositions were quench hardened and tempered and subjected to thermal TABLE 2 Number of cycles before thermal TRS 1 cracks X Materiel develop p.s.i.

(A) 4 250-300 (B) I 250-300 (C) 1 220-250 Invention 450550 1 Transverse Rupture Strength.

As will be noted from Table 2, the invention exhibits markedly superior resistance to thermal shock and markedly superior transverse rupture strength.

Material (B) in Table 2 was also compared with the composition of the invention as to impact resistance, the invention showing a value of 2.5 ft.-lbs. Charpy impact, while material (B) exhibited a Charpy impact value of 1.5 ft.-lbs., the invention being about 65 better than material (B).

It is important that the nickel content in the matrix not exceed 1% as too much nickel adversely atfects the strength properties. As stated hereinbefore, the rupture strength of the composition of the invention is over 325,000 p.s.i. and generally at least about 350,000 p.s.i., optimum values of 400,000 to 550,000 p.s.i. being attainable.

Thus, a composition containing by weight 25% TiC and 75 steel matrix, with the matrix consisting essentially by weight of 5% Cr, 4% Mo, 1% Ni, 0.4% C and the balance essentially Fe, exhibited a transverse rupture strength of 375,000 p.s.i. and a resistance to thermal shock corresponding to 18 repeated cycles of heating and cooling. Too much nickel (that is, above 1%) should be avoided since it is an austenite former and, therefore, tends to adversely affect the properties of the alloy.

It is also important that the amount of titanium carbide be controlled over the range of about to 30% by weight of the total composition. If the composition contains too much titanium carbide, the rupture strength falls off as does the resistance to thermal shock. Thus, a composition containing 35% TiC and 65% matrix (5% Cr, 4% Mo, 0.5% Ni, 0.4% C and the balance essentially iron) exhibited after quench hardening and tempering a transverse rupture strength of about 280,000 p.s.i. and a resistance to thermal shock corresponding to 5 repeated cycles of heating and cooling.

Summarizing the foregoing, the sintered composition of the invention provides a new and improved carbide steel composition which, when quench hardened and tempered, provides markedly improved properties compared to other broadly similar compositions described herein.

For example, the sintered composition when quenched from 1875 F. (1025 C.) exhibits a hardness of at least about 6 5 R and .when thereafter tempered at- 975 11 (525 C.) .for-l hour, a hardness of at least about 62 R .As already stated, the composition I generally exhibits transverse rupture properties in the hardened ;s tate ;-of over 325,000 p.s.i., preferably over 350,000 p.s.i. and usually from about 400,000 to 550,000 p.s.i. combined with improved resistance to thermal shock and room temperatureimpact. The alloy in'the shape of 1 inch x 1 inch x inchcan also berep'eatedly heated to l500- 'F; (815 C.) and quenched in oil for at least 10 times without cracking.

The alloy tends to resist softening at elevated temperatures and is suited for use as wear resistant elements, such as dies, elements involving sliding motion, apex seal strips for use in rotary piston engines and a multitude of other wear resistant applications.

Thus, the invention also provides as an article of manufacture, a hardened wear resistant element made of a steel-bonded carbide characterized by an improved combination of transverse rupture strength, resistance to impact and resistance to thermal shock, the composition comprising by weight about 20% to 30% of primary grains of TiC dispersed through a steel matrix constituting to 70% by weight, the composition of the matrix consisting essentially by weight of about 3%v to 7% chromium (preferably 4% to 6%), about 2% to 7% molybdenum (preferably 3% to 5%), about 0.1% to 1% nickel (preferably 0.25% to 0.75%), about 0.3% to 0.7% carbon (preferably 0.3% to 0.5%) and the balance of the matrix essentially iron, the steel matrix surrounding the primary carbide grains being characterized by a microstructure of essentially martensite.

Although the present invention has been described in conjunction with preferred embodiments, it is to be under stood that modifications and variations may be resorted to without departing from the spirit and scope of the invention as those skilled in the art will readily under-v stand. Such modifications and variations are considered to be within the purview and scope of the invention and the appended claims.

What is claimed is:

1. A sintered steel-bonded titanium carbide composition characterized by an improved combination of transverse rupture strength and resistance to thermal shock which comprises by weight about 20% to 30% of primary grains of titanium carbide dispersed through a steel matrix making up the balance, the composition of said matrix consisting essentially by weight of about 4% to 6% chromium, about 3% to 5% molybdenum, about 0.25 to 0.75% nickel, about 0.3% to 0.5 carbon and the balance essentially iron.

2. As an article of manufacture, a hardened wear resistant element made of a steel-bonded titanium carbide composition characterized by an improved combination of transverse rupture strength and resistance to thermal shock, said composition comprising by weight about 20% to 30% of primary grains of titanium carbide dispersed through a steel matrix making up the balance, the composition of said matrix consisting essentially by weight of about 4% to 6% chromium, about 3% to 5% molyb denum, about 0.25% to 0.75% nickel, about 0.3% to 0.5% carbon and the balance essentially iron, the steel matrix surrounding the primary carbide grains being characterized by a microstructure of essentially martensite.

3. The sintered compositionv of claim 1, consisting essentially by weight of approximately 25% by weight of TiC and approximately 75% by weight of the steel matrix, said matrix consisting essentially of about 5% chromium, about 4% molybdenum, about 0.5% nickel, about 0.4% carbon and the balance essentially iron.

8 4. The article of manufacture of claim wherein said 3,13 6,30" 6/ 1964 Cape et a1. V 7512 8 W composition consists essentially by weight of approxi- 3,109,917 11/1963 Schmidt et a1 219--76 mately 25% by weight of TiC and approximately 75% 2,828,202 3/1958 Goetlel et a1 148--126 by weight of the steel matrix, said matrix consisting essentially of about 5% chromium, about 4% molybdenum, CARL QUARFORTH, Primary Examiner about 0.5% nickel, about 0.4% carbon and the balance 6 SCHAFER Assistant Examiner essentially iron.

References Cited CL UNITED STATES PATENTS 1 126 3,653,982 4/1972 Prill 29-l82.8 X 10 3,416,976 12/1968 Brill-Edwards 148-126 X 

